This message was posted on behalf of Intel Corporation
You are correct, the difference between those parts is the low power feature of the 8265.NGWMG.S. The functionality and wireless features of both versions are equivalent; however, your computer may support one version, the other, or none, depending on how it was designed by the Computer Manufacturer.
If you are planning to upgrade the wireless adapter in your PC with the Intel® Dual Band Wireless-AC 8265, then we strongly advise to engage the Computer Manufacturer Support, and have it replaced with their authorized service provider if possible. Intel does not recommend end users install or upgrade the various Intel® Wireless Adapter.
Using an adapter that is not supported by the PC may lead to errors, system damage, or it may even be illegal. Please review the following documents for more information:
Regulatory Information Regarding Hardware Installation or Upgrade
Why Doesn't My Laptop Recognize My New Intel® Wireless Adapter?
The AC 8265 is a great wireless adapter, and as you mentioned, desktop systems normally use the regular versions (not the low power ones), however, the system manufacturer is the only one that can confirm if it is supported in your computer.